Interference suppression

9.8.2 Interference suppression

The ignition system of a vehicle is the largest The process of interference suppression on a

source of interference, in particular the high ten- vehicle is to reduce the amount of unwanted

sion side. Voltages up to 30 kV are now common noise produced from the speakers of an ICE sys-

and the current for a fraction of a second when tem. This can, however, be quite difficult. To aid

the plug fires can peak in excess of 100 A. The the discussion it is necessary to first understand

interference caused by the ignition system is the different types of interference. Figure 9.31

mostly above 30 MHz and the energy can peak shows two signals, one clean and the other suf-

for fractions of a second in the order of 500 kW. fering from interference. The amount of interfer-

The charging system produces noise because of ence can be stated as a signal to noise ratio. This

the sparking at the brushes. Electronic regulators is the useful field strength compared to the inter-

produce few problems but regulators with vibrat- ference field strength at the receiver.

ing contacts can cause trouble.

Electrical systems 221

Any motor or switch including relays is likely the interference field of the vehicle. For reception to produce some interference. The most popular

in the AM bands the aerial represents a capaci- sources are the wiper motor and heater motor. The

tance of 80 pF with a shunt resistance of about starter is not considered due to its short usage time.

1 M⍀. The set will often incorporate a trimmer Buildup of static electricity is due to friction

to ensure that the aerial is matched to the set. between the vehicle and the air, and the tyres and

Contact resistance between all parts of the aerial the road. If the static on say the bonnet builds up

should be less than 20 m⍀. This is particularly more than on the wing then a spark can be dis-

important for the earth connection. charged. Using bonding straps to ensure all

When receiving in the FM range the length of panels stay at the same potential easily prevents

the aerial is very important. The ideal length of a this. Due to the action of the tyres a potential can

rod aerial for FM reception is one quarter of the build up between the wheel rims and the chassis

wavelength. In the middle of the FM band (94 MHz) unless suitable bonding straps are fitted. The arc

this is about 80 cm. Due to the magnetic and elec- to ground can be as much as 10 kV.

trical field of the vehicle and the effect of the There are five main techniques for suppress-

coaxial cable, the most practical length is about ing radio interference:

1 m. Some smaller aerials are available but whilst

these may be more practical the signal strength is resistors;

reduced. Aerials embedded into the vehicle win- bonding;

dows or using the heated rear window element are screening;

good from the damage prevention aspect and for capacitors;

insensitivity to moisture, but produce a weaker inductors. signal often requiring an aerial amplifier to be

Resistance is used exclusively in the ignition HT included. Note that this will also amplify interfer- circuit, up to a maximum of about 20 k ⍀ per lead.

ence. Some top range vehicles use a rod aerial and This has the effect of limiting the peak current,

a screen aerial, the set being able to detect and which in turn limits the peak electromagnetic radi-

use the strongest signal. This reduces the effect ation. Providing excessive resistance is not used,

of reflected signals and causes less flutter. the spark quality is not affected. These resistors

Consideration must be given to the position of effectively damp down the interference waves.

an external aerial. This has to be a compromise Bonding has been mentioned earlier. It is sim-

taking into account the following factors: ply to ensure that all parts of the vehicle are at the

1 m if possible; same electrical potential to prevent sparking due ● coaxial cable length longer cable reduces to the buildup of static.

● rod length

the signal strength;

Screening is generally only used for specialist ● position as far away as reasonably possible applications such as emergency services and the

from the ignition system; military. It involves completely enclosing the igni- ● potential for vandalism out of easy reach; tion system and other major sources of noise in ● aesthetic appearance does it fit with the

a conductive screen, which is connected to the

style of the vehicle;

vehicle’s chassis earth. This prevents interference ● angle of fitting vertical is best for AM hor- waves escaping, it is a very effective technique but

izontal for FM!

expensive. Often a limited amount of screening, metal covers on the plugs for example, can be

Most quality sets also include a system known as used to good effect.

interference absorption. This is a circuit built in Capacitors and inductors are used to act as fil-

to the set consisting of high quality filters and is ters. This is achieved by using the changing value

not adjustable.

of ‘resistance’ to alternating signals as the fre- quency increases. The correct term for this resist- ance is either capacitive or inductive reactance. By choosing suitable values of capacitor in paral-

9.9 Body electrical

lel and/or inductor in series it is possible to filter

systems

out unwanted signals of certain frequencies. The aerial is worth a mention at this stage.